8milelake 131 Piece Helicoil Type Thread Repair Kit - M5 M6 M8 M10 M12

Q: Do they make this kit in standard ( SAE ) as well ?

Yes, you can buy individual inserts as well

Q: Will it work on transmission holes

It's for repairing damaged/stripped threads in metal. If you need to repair a stripped hole that accepts a metric bolt with the same thread pitch in the description, then yes, it will work.

Q: would this work on a Yamaha 250 CC spark plug hole?

You need to know the original size of the hole so you can go a size bigger. This set is very good worked great for what I needed.

Q: How many helicoils there are per thread size?

Depends on the size. More for the smaller, less for the larger but plenty to last a good long time. Maybe a dozen for the largest? This is a good quality kit which saves my bacon on the regular.